The prognosis with Lymphoma as you can probably guess is not good, there is no cure. I am so sorry, we have been through it wit one of our girls.
We noticed Treacle had lost weight and found a lump under her chin. Upon closely inspection at the vets our vet showed us several other lumps around lymph nodes on piggies legs etc... Rosie was certain it was Lymphoma and told us the treatment options. We could slow the progress down with steroids or try Chemo. There is no way we could put an animal through Chemotherapy so we choose to try and give Treacle a little longer with us by steroid injections.
Rosie assured us that the Lymphoma Treacle had was causing her no pain and up until the night she passed away she was a very happy content piggy (apart from the few episodes of bloat) From diagnosis till Treacy passed was around 1 month, but her lumps where quite big.
I would certainly recommend steroid treatment via injections think Treacy had two shots. Sides effects are they lower immune system but we had no issues with infections from what I recall. So vet needs to keep eye on this.
Also means they can’t have standard pain relief in metacam as it is anti inflammatory. Our vet prescribed an opiate for pain releif as treacle had a bladder issue and most importantly emeprid was prescribed also as opiates can slow guts down.
Most important thing is keeping them pain free for as long as possible before helping them across the bridge when they need it, please discuss this with the vet so you know what to look out for when the time comes.
Appetite is higher as well, we feed her extra each day

It was a very bitter sweet time as Treacy got used to coming out for extra feed and would sit on my chest eating hay when I lay on the floor with her
Here is a link to Treacle's thread, I kept it updated with her progress....
Treacle has Lymphoma as you can guess it doesn't end happily. But I wouldn't have changed a thing with her treatment wise.
I am so sorry again
Lee